CHEMICAL BONDING AND MOLECULAR GEOMETRY

 

GEOMETRY OF MOLECULES:

Covalent molecule cane be expressed in many ways. For example: Molecular Formula, Condensed Formula, Structural Formula, Lewis Dot structure etc

None of these determines geometry of molecule. In order to deduce geometry of small covalent molecules, Several Theories were purposed: They are:

• VSEPR Theory
• VBT Theory
• MOT

VSEPR Theory:

VSEPR theory was purposed by Sidgwick and Powell and was modified by Gillespie and Nyholm. It predicts geometry of small covalent molecule. The postulates of this theory are as follows:

1.Atoms arrange around central atom such a way that there exist minimum repulsion between electron pair at valence shell of central atom to attain maximum stability and minimum energy.

2.The number of electron pairs at valence shell of central atom determine geometry of molecule. Molecule having 2,3,4,5,6 and 7 electron pairs at valence shell of central atom have linear, trigonal planar, tetrahedral,, trigonal bipyramidal, square bipyramidal (octahedral), pentagonal bipyramidal respectively.

3.Presence of lone pair at central atom distort geometry this is because L.P-L.P>L.P-B.P>B.P-B.P repulsion. This is because:

• Lone pair of electrons concentrate at single atom.
• Bond pair of electrons dispersed between 2 atoms.
  
  
  

4.The magnitude of repulsion between electron pairs depend on electronegativity difference between central atom and atom attached to it.

• If central atom is more electronegative than atom attached to it, bond angle increases.
• If central atom is less electronegative than atom attached to it, bond angle decreases.
  
  
  

5.Only electron pair involved in sigma bond formation determines geometry of molecule.

Application of VSEPR Theory

Regular geometry

1)BeCl₂

In BeCl₂, Be atom has two electron pairs at its valence shell so it takes linear geometry with bond angle '180˚' so that there exists minimum repulsion between valence electron pairs at Be atom to attain minimum energy and maximum stability according to VSEPR theory.

2)BF₃

In BF₃, B atom has three electron pairs at its valence shell so it takes trigonal planar geometry with bond angle '120˚' so that there exists minimum repulsion between valence electron pairs at B to attain minimum energy and maximum stability according to VSEPR theory.

3)CH₄

In CH₄, C atom has four electron pairs at its valence shell so it takes tetrahedral geometry with bond angle 109˚28' so that there exists minimum repulsion between valence electron pairs at C atom to attain minimum energy and maximum stability according to VSEPR theory.

4)PCl₅

In PCl5, P atom has five electron pairs at its valence shell so it takes trigonal bipyramidal geometry with bond angle 120˚ and 90˚ so that there exists minimum repulsion between valence electron pairs at P atom to attain minimum energy and maximum stability according to VSEPR theory.

5)SF₆

In SF6, S atom has six electron pairs at its valence shell so it takes square bipyramidal geometry (octahedral) with bond angle 90˚ so that there exists minimum repulsion between valence electron pairs at S atom to attain minimum energy and maximum stability according to VSEPR theory.

6)IF₇

In IF7, I atom has seven electron pairs at its valence shell so it takes pentagonal bipyramidal geometry with bond angle 72˚ and 90˚ so that there exists minimum repulsion between valence electron pairs at I atom to attain minimum energy and maximum stability according to VSEPR theory.

Irregular geometry

1)NH₃

Ammonia consists of one lone pair and three bond pairs of electrons. Due to the presence of lone pair,  there is distortion of geometry. Since lone-bond pair > bond pair-bond pair repulsion,  the bond angle reduces to 107°.

Thus the spatial arrangement of four valence electron pair is distorted tetrahedral and the geometry is pyramidal for ammonia.

2)H₂O

It consists of two lone pair and two bond pairs of electrons. Due to the presence of lone pairs electrons,  there is exists appreciable distortion of geometry. Since lone-lone pair>lone-bond pair > bond pair-bond pair repulsion,  the bond angle reduces to 104.5°.

Thus the spatial arrangement of four valence electron pair is distorted tetrahedral and the geometry is angular for water.

Comparison of bond angle of CH_4, NH₃and H₂O

Due to only bond pairs of electrons in methane,  it exists in tetrahedral geometry with bond angle 109°28'.

In contrary,  ammonia has lone pair in addition to three bond pairs of electrons. Due to lone pair-bond pair>bond-bond pair repulsion, there is reduction in bond angle 109°28' to 107°.

Similarly,  it has two lone pair of electrons in water. So there exists greater reduction in bond angle to 104.5° due to Lone pair-lone pair >lone pair-bond pair> bond pair-bond pair repulsion.

Comparison of bond angle of H₂S and H₂O

Water and hydrogen sulphide differsin the central atom. Oxygen being more electronegative than sulphur atom. That is why repulsion between bond pairs of electrons in water is higher than that of hydrogen sulphide.

Comparison of bond angle of NH₃and PH₃

Ammonia and phosphine differ in the central atom. Nitrogen being more electronegative than phosphorous atom. That is why repulsion between bond pairs of electrons in ammonia is higher than that of phosphine.

Comparison of bond angle of NH₃and NCl₃

Ammonia and nitrogen trichloride differ in the terminal atoms. Chlorine being more electronegative than hydrogen atom. That is why repulsion between bond pairs of electrons in ammonia is higher than that of nitrogen trichloride because chlorine pulls away the electro pair from central in comparision to hydrogen.

Valence Bond Theory

Hybridization

Definition

Criteria for hybridization

Characteristics of hybridization

Types of hybridization

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This site provides notes on different chapters of Chemistry of Grade XI and XII (NEB Board). Also, it posts number of blogs on Collection of past questions and solution. Conversion chart, rules, Chemical test are some additional features.

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